This invention concerns a method of joining forged, machined, formed or cast components with metal tube. In another aspect it concerns a metal object constructed according to the method. The invention is particularly, but not exclusively, applicable to the construction of vehicle wheels having a forged, machined, formed or cast hub or center and a metal rim.
A large number of techniques are available for the construction of objects such as fabricated car wheels, which require the joining of a circular rim component with a center or hub component. Welding is the most common form of joining these parts, but riveting, bolting, and bonding with adhesives are methods also used. Electric welding generate fumes, can distort the parts, can degrade the metallurgy of the parts, and requires high energy inputs and similarity of the metals to be jointed. Friction welding also requires similarity of the metals, expensive technology and machinery which imposes heavy loads on the parts during the joining process. Riveting and bolting methods require the provision of mating flanges which impose styling limitations and add weight to the wheel. The rivet and bolt holes are also points from which cracks can propagate and, particularly with bolted wheels, there is always the risk that the bolts are not tightened such that their performance is totally reliable in extended use. Bonding as the primary means of joining wheel components has not, to date, proved to have market acceptance, following a history of failures in the past.
The invention provides a method of joining forged, machined, formed or cast central components with metal tube. The method comprises the steps of:
heating the tube to expand its diameter, but not sufficiently to anneal it, or otherwise adversely affect its mechanical properties.
Then, while the tube is still hot, pushing the tube over part of the central component so that part of the tube overlies a circumferential groove in the central component.
Then, while still hot, rolling the circumferential part of the tube which overlies the groove, into the groove, so that when cooled the rolled part of the tube is clamped into the groove, and another part is shrunk on to the exterior of the central component.
This process may be cheap, and robust, giving reliable results which don""t degrade the metallurgy of the joint. It has marketing credibility and offers freedom of styling.
As a preliminary step, the circumferential groove may be machined into the hub or center component.
Where an end of the metal tube is to be rolled into the circumferential groove in the central component, then a stop may be provided along one side of the groove. The heated tube may be pushed over the component from the other side until the end of the tube abuts the stop.
The end of the tube abutting the stop may be bevelled or knurled. After the end of the tube has been rolled into the groove, the stop may be spin forged, while the tube is still hot, into the space remaining in the groove as a result of the bevelling of the end of the tube. This gives a neat appearance and a smooth finish. The spin forged center material interlocks with the knurled tube end, providing reliable torque transmission capability for the joint. It also locks the joint against any axial movement and any tendency of the joint and of the rim to open up under load. It also provides a practical way of removing the stop and improving the appearance of the joint without recourse to a secondary operation.
The joint system relies for its integrity on the fact that the rolling and spin forging operations are done hot, that when the whole joint cools, the shrinkage of the formed metal in the joint, relatively to the center component provides a tight uniform clamping force with a high degree of pre-load.
It is important that the metal tube is heated only sufficiently to increase its ductility, or formability, but not enough to anneal the material or otherwise adversely affect its mechanical properties. For some materials, such as mild steel, which possesses very good formability in the as-supplied condition, heating is not required for any purpose other than to introduce the shrinkage factor into the joint.
Adhesive may be applied to the mating surfaces of the joint prior to assembly to provide a reliable torque transmission capability, and also to seal the joint.
As an alternative, or in addition to bevelling or knurling the end of the tube, dimples may be provided in the floor of the groove into which the tube is rolled in order to provide a mechanical interlock. This may be particularly useful where no stop is employed and the alloy tube is pushed entirely over the central component to extend from either side.
The operation may be applied to the fabrication of a vehicle wheel. In this case, the central component may be a cast, machined, formed or forged hub, and the metal rim of the wheel may comprise a formed tubular structure which is pushed on to the hub. The rim may be pushed on to the hub for a predetermined distance, or pushed right over the hub to extend either side by the desired amounts before the joining process is completed.
In another aspect, the invention provides a vehicle wheel comprising a central cast, machined, formed or forged hub or center, and a metal rim connected to the hub by being shrunk fit on to it and having at least one part roll formed down into a circumferential groove around the hub or center.
In addition a stop formed in part of the hub may be spin forged down over the top of the roll formed part of the rim. If the rim is bevelled or knurled before roll forming, then a space will be left afterwards into which the stop may be spin forged. Also in addition, a sealing material may be provided between the rim and the hub in order to improve the bonding, or seal the joint, or both. Dimples may also be introduced into the bed of the groove to provide a mechanical interlock when the rim is rolled into the groove.
The method is particularly suitable for the construction of the vehicle wheels due to the homogenous 360 degree clamping and locking affect which is able to be produced. In addition, the method provides for the isolation of the critical locking portion of the joint, that is the roll into the groove, by the shrink fit of the rim on to the central component.